Growth crystallography of directionally solidified FeNbNi eutectic alloy

METALLOGRAPHY 13:379-381 (1980)

379

Growth Crystallography of Directionally Solidified Fe-Nb-Ni Eutectic Alloy

SURENDRA N. TEWARI

Defence Metallurgical Research Laboratory,, PO Kanchanbagh, Hyderabad 500 258. India

Jaffrey and Marich [I] investigated a directionally solidified Fe-13.3 Nb-23 Cr (w/o) eutectic alloy. The alloy consisted of 22% hcp intermetallic phase, e ({-Fe2Nb type), in a bcc iron-chromium solid solution

Fro.~ I, Microstructure of directionally solidified Fe-16.5 Nb-15,4 Ni alloy, showing the a - { eutectic (a matrix partially etched out to reveal e plates).

© Elsevier North Holland, Inc., 1980 52 Vanderbilt Ave., New York, NY 10017

0026-0800/80/040379 + 3501.75

S. N. Tewari

380

(a)

(b) FIG. 2. Electron diffraction patterns taken on a plane perpendicular to the F e - 1 6 , 5 N b - 1 5 . 4 Ni alloy growth direction (dotted line is e¢-~ interface, rotated by 10° to c o m p e n s a t e for image rotation), (a) In c~ phase. (b) In ~ phase.

Short Communication

381

matrix. They, however, could not obtain the plane front solidification condition and, therefore, produced an alloy with a cellular microstructure. The intermetallic phase adopted a fibrous growth morphology except at cell boundaries where it had a plate-like morphology. They were not successful in preparing thin specimens suitable for transmission electron microscopy and therefore could not investigate the growth crystallography. This communication reports the growth crystallography observed in a similar directionally solidified eutectic alloy. The alloy, with a nominal composition of Fe-16.5 Nb-15.4 Ni (w/o), was directionally solidified in a recrystallized alumina tube in a modified Bridgeman furnace [2]. Aligned lamellar microstructure was obtained at a growth speed of 0.5 cm/hr with the temperature gradient in the liquid-solid interface of about 200°C/cm. Figure I shows a scanning electron microscope edge view of the microstructure with the matrix partially etched out to reveal the e-intermetallic plates. The microstructure consists of about 41 volume % of hcp intermetallic lamellae (e), embedded in a bcc matrix (a). Thin slices cut transverse to the alloy growth direction by electrondischarge machining were mechanically reduced to about I00 ixm thickness by polishing on a fine emergy paper. These were further thinned chemically, using an equal volume solution of H20, H2Oz, HF, and HNO3, to about 40 txm thickness. These disks, when electropolished tc perforation at - 80°C in a 0.75% perchloric acid/alcohol solution, provided the specimens suitable for transmission electron microscopy. The electron diffraction patterns obtained in the a phase and in the phase near the a - e interface are shown in Fig. 2(a) and (b), respectively. The orientation relationship between the two phases and the alloy growth direction obtained from this figure can be expressed as [lll]= II [001]~ growth direction and (112)~ II (010), at the a - e interface. This work was carried out when the author was a National Research Council Associate at NASA-Lewis Research Center, Cleveland (USA). Appreciation is expressed to Mr. Bruno Buzek for his assistance with the electron microscopy.

References 1. D. Jaffrey and S. Marich, An F e - C r - N b pseudo-binary eutectic alloy, Met. Trans. 3: 551-558 (1972). 2. S. N. Tewari, Directionally solidified iron-based eutectic superalloys, NASA TND-8354 (1976).

Received April 1980: accepted July 1980.